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如果你安装的Visual Studio，以及它的Visual C++的话，
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那么在安装目录下的VC/crt/src下有所有标准C库的源代码
另外，h后缀的头文件包含函数的声明，具体的实现都在c后缀的源码文件中
1）字符串操作
strcpy(p, p1) 复制字符串
strncpy(p, p1, n) 复制指定长度字符串
strcat(p, p1) 附加字符串
strncat(p, p1, n) 附加指定长度字符串
strlen(p) 取字符串长度
strcmp(p, p1) 比较字符串
strcasecmp忽略大小写比较字符串
strncmp(p, p1, n) 比较指定长度字符串
strchr(p, c) 在字符串中查找指定字符
strrchr(p, c) 在字符串中反向查找
strstr(p, p1) 查找字符串
strpbrk(p, p1) 以目标字符串的所有字符作为集合，在当前字符串查找该集合的任一元素
strspn(p, p1) 以目标字符串的所有字符作为集合，在当前字符串查找不属于该集合的任一元素的偏移
strcspn(p, p1) 以目标字符串的所有字符作为集合，在当前字符串查找属于该集合的任一元素的偏移
* 具有指定长度的字符串处理函数在已处理的字符串之后填补零结尾符
2）字符串到数值类型的转换
strtod(p, ppend) 从字符串 p 中转换 double 类型数值，并将后续的字符串指针存储到 ppend 指向的 char* 类型存储。
strtol(p, ppend, base) 从字符串 p 中转换 long 类型整型数值，base 显式设置转换的整型进制，设置为 0 以根据特定格式判断所用进制，0x, 0X 前缀以解释为十六进制格式整型，0 前缀以解释为八进制格式整型
atoi(p) 字符串转换到 int 整型
atof(p) 字符串转换到 double 符点数
atol(p) 字符串转换到 long 整型
3）字符检查
isalpha() 检查是否为字母字符
isupper() 检查是否为大写字母字符
islower() 检查是否为小写字母字符
isdigit() 检查是否为数字
isxdigit() 检查是否为十六进制数字表示的有效字符
isspace() 检查是否为空格类型字符
iscntrl() 检查是否为控制字符
ispunct() 检查是否为标点符号
isalnum() 检查是否为字母和数字
isprint() 检查是否是可打印字符
isgraph() 检查是否是图形字符，等效于 isalnum() | ispunct()
很遗憾,标准库中的函数结合了系统,硬件等的综合能力,是比较近机器的功能实现,所以大部分是用汇编完成的,而且已经导入到了lib和dll里了,就是说,他们已经被编译好了,似乎没有代码的存在了.
能看到的也只有dll中有多少函数被共享.
第三方可能都是dll,因为上面也说了,dll是编译好的,只能看到成品,就可以隐藏代码,保护自己的知识产权,同时也是病毒的归宿...... 当然,除了DLL的确还存在一种东西,插件程序~~~
C语言标准库函数
标准io函数
Standard C I/O
clearerr() clears errors
fclose() close a file
feof() true if at the end-of-file
ferror() checks for a file error
fflush() writes the contents of the output buffer
fgetc() get a character from a stream
fgetpos() get the file position indicator
fgets() get a string of characters from a stream
fopen() open a file
fprintf() print formatted output to a file
fputc() write a character to a file
fputs() write a string to a file
fread() read from a file
freopen() open an existing stream with a different name
fscanf() read formatted input from a file
fseek() move to a specific location in a file
fsetpos() move to a specific location in a file
ftell() returns the current file position indicator
fwrite() write to a file
getc() read a character from a file
getchar() read a character from STDIN
gets() read a string from STDIN
perror() displays a string version of the current error to STDERR
printf() write formatted output to STDOUT
putc() write a character to a stream
putchar() write a character to STDOUT
puts() write a string to STDOUT
remove() erase a file
rename() rename a file
rewind() move the file position indicator to the beginning of a file
scanf() read formatted input from STDIN
setbuf() set the buffer for a specific stream
setvbuf() set the buffer and size for a specific stream
sprintf() write formatted output to a buffer
sscanf() read formatted input from a buffer
tmpfile() return a pointer to a temporary file
tmpnam() return a unique filename
ungetc() puts a character back into a stream
vprintf, vfprintf, vsprintf write formatted output with variable argument lists
标准字符/字符串处理函数
atof() converts a string to a double
atoi() converts a string to an integer
atol() converts a string to a long
isalnum() true if alphanumeric
isalpha() true if alphabetic
iscntrl() true if control character
isdigit() true if digit
isgraph() true if a graphical character
islower() true if lowercase
isprint() true if a printing character
ispunct() true if punctuation
isspace() true if space
isupper() true if uppercase character
isxdigit() true if a hexidecimal character
memchr() searches an array for the first occurance of a character
memcmp() compares two buffers
memcpy() copies one buffer to another
memmove() moves one buffer to another
memset() fills a buffer with a character
strcat() concatenates two strings
strchr() finds the first occurance of a character in a string
strcmp() compares two strings
strcoll() compares two strings in accordance to the current locale
strcpy() copies one string to another
strcspn() searches one string for any characters in another
strerror() returns a text version of a given error code
strlen() returns the length of a given string
strncat() concatenates a certain amount of characters of two strings
strncmp() compares a certain amount of characters of two strings
strncpy() copies a certain amount of characters from one string to another
strpbrk() finds the first location of any character in one string, in another string
strrchr() finds the last occurance of a character in a string
strspn() returns the length of a substring of characters of a string
strstr() finds the first occurance of a substring of characters
strtod() converts a string to a double
strtok() finds the next token in a string
strtol() converts a string to a long
strtoul() converts a string to an unsigned long
strxfrm() converts a substring so that it can be used by string comparison functions
tolower() converts a character to lowercase
toupper() converts a character to uppercase
标准数学函数
abs() absolute value
acos() arc cosine
asin() arc sine
atan() arc tangent
atan2() arc tangent, using signs to determine quadrants
ceil() the smallest integer not less than a certain value
cos() cosine
cosh() hyperbolic cosine
div() returns the quotient and remainder of a division
exp() returns "e" raised to a given power
fabs() absolute value for floating-point numbers
floor() returns the largest integer not greater than a given value
fmod() returns the remainder of a division
frexp() decomposes a number into scientific notation
labs() absolute value for long integers
ldexp() computes a number in scientific notation
ldiv() returns the quotient and remainder of a division, in long integer form
log() natural logarithm
log10() natural logarithm, in base 10
modf() decomposes a number into integer and fractional parts
pow() returns a given number raised to another number
sin() sine
sinh() hyperbolic sine
sqrt() square root
tan() tangent
tanh() hyperbolic tangent
标准时间/日期函数
asctime() a textual version of the time
clock() returns the amount of time that the program has been running
ctime() returns a specifically formatted version of the time
difftime() the difference between two times
gmtime() returns a pointer to the current Greenwich Mean Time
localtime() returns a pointer to the current time
mktime() returns the calendar version of a given time
strftime() returns individual elements of the date and time
time() returns the current calendar time of the system
标准内存管理函数
calloc() allocates a two-dimensional chunk of memory
free() makes memory available for future allocation
malloc() allocates memory
realloc() changes the size of previously allocated memory
其它标准函数
abort() stops the program
assert() stops the program if an expression isn';t true
atexit() sets a function to be called when the program exits
bsearch() perform a binary search
exit() stop the program
getenv() get enviornment information about a variable
longjmp() start execution at a certain point in the program
qsort() perform a quicksort
raise() send a signal to the program
rand() returns a pseudorandom number
setjmp() set execution to start at a certain point
signal() register a function as a signal handler
srand() initialize the random number generator
system() perform a system call
va_arg() use variable length parameter lists
标准库只是定义接口，具体怎么实现就得看操作系统，你说win下和linux下这些函数的实现会一样吗。当然不一样，看这些学源码，不如看看c标准，c89或c99.
那可以看内核，看系统调用是怎么样实现的，你说的那些都是基于系统调用的
你说的就是库函数的源码，也就是glibc，源码在可以下到，比如下载，打开后就可以看到你需要的各种库的具体实现代码，比如在string中的strcat.c中就有
char *strcat (dest, src)
char *dest;
const char *src;
{
char *s1 = dest;
const char *s2 = src;
reg_char c;
/* Find the end of the string. */
do
c = *s1++;
while (c != '\0');
/* Make S1 point before the next character, so we can increment
it while memory is read (wins on pipelined cpus). */
s1 -= 2;
do
{
c = *s2++;
*++s1 = c;
}
while (c != '\0');
return dest;
}